1. Field of the Invention
The present invention relates generally to a transmission line driver and methods thereof, and more particularly to a transmission line driver for controlling slew rate and methods thereof.
2. Description of the Related Art
In a system requiring a higher-speed signal transmission, a transition time of an output signal may be a factor in determining the characteristics of a semiconductor integrated circuit. When the transition time of an output signal is less than a first threshold, Electromagnetic Interference (EMI) and/or switching noise may increase. Alternatively, when the transition time of an output signal is higher than a second threshold (e.g., where the second threshold is longer than the first threshold), jitter may be generated. Accordingly, the transition time of an output signal may be set in order to avoid the above-described timing problems.
FIG. 1 is a timing diagram illustrating the concept of a slew rate. As shown in FIG. 1, the slew rate may refer to a voltage rate of change of an output signal during a transition from a first logic level (e.g., a higher voltage level) to a second logic level (e.g., a lower voltage level) or from the second logic level to the first logic level in response to an input signal VIN. The input signal VIN may be a square wave.
FIG. 2 is a schematic diagram illustrating a conventional push-pull type transmission line driver 200. Referring to FIG. 2, the push-pull type transmission line driver 200 may include a slew-rate control circuit 21 to control the voltage applied to the gates of the pull-up transistor MP1 and/or a pull-down transistor MN1 such that the transmission line driver 200 may control a charging time and/or a discharging time of the output signal of the push-pull transmission line driver 200. However, the push-pull type transmission line driver 200 may not be suitable for use in a system operating at higher speeds because the output voltage VOUT may oscillate between a supply voltage VDD and a ground voltage GND.
FIG. 3 is a schematic diagram illustrating a conventional transmission line driver 300 including a pull-up resistor RPU. Referring to FIG. 3, the transmission line driver 300 may control the slew rate by adjusting a voltage received at the gate of a driving transistor MN2. The transmission line driver 300 may adjust the voltage applied to the gate of the driving transistor MN2 with a slew-rate control circuit 31.
An output voltage VOUT of the transmission line driver 300 may not make a full transition (e.g., between the first voltage level and the second voltage level and/or the second voltage level and the first voltage level) because the pull-up resistor RPU and the current source IS1 may be coupled between the supply voltage VDD and the ground voltage GND. Thus, when a voltage applied to the gate of the driving transistor MN2 is changed to adjust the slew rate, a swing width (e.g., the time required to make a complete voltage level transition) of the output voltage VOUT and/or the slew rate may be changed. Further, the slew-rate control circuit 31 may be a complicated circuit (e.g., difficult and/or expensive to fabricate).